In addition to the phosphorus-containing groups of side-reaction products described in U.S. Pat. No. 3,192,242, it has been found that the following phosphorus-containing group is also formed: ##EQU2## For example, the following compound has been specifically identified: ##EQU3##
In the reaction of the pentaerythritol ester of phosphorohalidous acid to produce flame retardant materials, i.e. by the subsequent reaction of the pentaerythritol ester of phosphorohalidous acid with halogen and, if desired, epoxy compounds as disclosed in U.S. Pat. No. 3,192,242, it has been discovered that such flame retardant materials are of inferior quality due to variations in the viscosity of the products obtained and because of a variation in the concentration of low boiling components, formed from the side-reaction products, contained therein. In order to function properly as a flame retardant, the flame retardant product must be of relatively constant viscosity and contain as low a concentration as possible of low boiling components. Further, it has become clear that purification of the pentaerythritol ester of phosphorohalidous acid, prior to subsequent reactions to produce a flame retardant, lowers the yield, thereby raising the cost, and does not conveniently eliminate the viscosity and low boiling component variations of subsequent reaction products.